Field of the Invention
The present invention relates to a method for preparing a visible light-responsive photocatalyst and a visible light-responsive photocatalyst intermediate, a usage of a visible light-responsive photocatalyst, and the visible light-responsive photocatalyst.
Related Art
Numerous products have recently been developed by utilizing an oxidation-reduction reaction with a semiconductor photocatalyst. There have been developed practical products for everyday life, for example, tunnel lightings which exhibit a self-cleansing action due to a photocatalyst applied, tiles which exhibit an antibacterial action due to a photocatalyst added, clothes made by knitting fibers mixed with a photocatalyst, and the like.
Most of photocatalysts used in these products are titanium oxides. Titanium oxide is widely used because of the following reasons. That is, titanium oxide exhibits extremely high catalytic activity upon irradiation with light and also does not dissolve in water or an organic solvent; and titanium oxide is stable, non-toxic, comparatively inexpensive, and has a plenitude of natural resources.
Brookite-type, anatase-type, rutile-type, and the like are known as crystal forms of titanium oxide. In order to enable the titanium oxide to exhibit a photocatalytic function, anatase-type requires ultraviolet light of 380 nm or less, and rutile-type requires ultraviolet light of 400 nm or less. However, main light constituting sunlight is visible light having wavelength of 400 nm to 800 nm, and the amount of ultraviolet light of 380 nm or less required for titanium oxide to exhibit a photocatalytic function only accounts for about 3% of the total light. That is, the amount of ultraviolet light of sunlight is a very small amount as light which enables exhibition of a photocatalytic function of titanium oxide. Main light of a fluorescent lamp is also visible light having a wavelength of 400 nm to 650 nm.
In light of these circumstances, there is an urgent need to develop a visible light-responsive catalyst which does not respond to ultraviolet light which only exists in a small amount, but respond to visible light which can be gained in a large amount, leading to activation.
There has been found, as a visible light sensitization method of titanium oxide, a method in which composite fine particles of gold colloids and titanium oxide fine particles are formed and plasmon-mediated light emission of gold colloids are utilized. There has also been developed, as a method for preparing composite fine particles of gold colloids and titanium oxide, a method in which composite fine particles are prepared by a reverse micelle method (see Patent Document 1).    [Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2005-342601